In the process of acquiring seismic data, it is conventional to use a seismic vibrator to input seismic energy into the ground. Seismic energy is generally applied over time where the vibrators begin a sweep by vibrating initially at a low frequency and progressively increase the frequency such that an entire sweep of the frequency range is delivered within a definite time. Sweeps of four to eight seconds have been standard practice for years, but longer sweeps are becoming increasingly common with sixteen second sweeps and twenty four second sweeps also being used.
The costs for a seismic survey can be quite expensive and much effort has gone into improving the efficiency of seismic surveying. One advance is to operate several seismic vibrators at the same time all making a similar sweep, but at different phases with respect to one another. In other words, if the baseplate of one vibrator were to be going up while another is going down, the two vibrators would be about 180 degrees out of phase. Operating four vibes that are out of phase with respect to one another is known and commercially in use as the HFVS or ZenSeis® geophysical prospecting system among others. Thus, commonly four (or some other number of) vibrators can be delivering seismic energy at one time and are each identifiable in the recordings from all of the seismic receivers. Typically, with four vibrators, at least four separate sweeps are performed where the phase relationship between the vibrators is changed between sweeps to enhance the distinctiveness of each vibrator in the data record.
It has also been recognized that being out of phase at an orthogonal relationship to one another is not the most distinctive. If one vibrator is at zero degrees phase and the next vibrators is 90 degrees ahead, the next is 180 degrees ahead and the last is 270 degrees ahead, this combination is described as orthogonal where everything is 90 or 180 degrees different from one another. In this arrangement, echoes and harmonics from the subsurface geological structures are created and are somewhat difficult to distinguish from the principal reflections. So, it is preferred that phase differences are non-orthogonal which makes the phase differences between at least two of the four vibrators to be less than 90 degrees. Unfortunately, equipment and circumstances are never perfect and vibrators that are supposed to be out of phase may actually operate at a phase difference that becomes difficult to distinguish due to equipment drift or wear and tear. In this circumstance, it may not be apparent to the operators that the phase excursion has happened until after much or all of the survey is completed. The cost of re-running the survey or the portions of the survey may not be justifiable.